(CNN) -- Military officials are investigating whether the horrific attack that killed four U.S. civilian contractors in Fallujah, Iraq, this week was planned.

The normally busy streets in the city about 30 miles west of Baghdad were empty and shops were closed at the time of the attack, the officials told CNN.

Also, a number of local Iraqi media were in the area.

Officials said there is no indication that residents of Fallujah knew what was being planned, but there was talk on the street that trouble might erupt.

Military investigators are also looking for any evidence that Abu Musab Zarqawi may have had a hand in the attack.

Abu Musab al-Zarqawi, a purported al Qaeda operative, is blamed by Washington for organizing several attacks against Iraqi civilians and coalition targets in Iraq.

U.S. Marines are expected to re-enter Fallujah soon in a move to take control.

Fallujah is part of al Anbar province in the Sunni Triangle, a region north and west of the capital that has been a hotbed of opposition to the U.S. presence.

Witnesses said the four Americans were ambushed Wednesday after leaving a military base east of Fallujah and making their way into the city in two sport utility vehicles.

As they were making a turn on a Fallujah street, a group of men, with faces covered by headscarves, split into two groups and threw hand grenades into the vehicles.

The two vehicles were engulfed in flames as the assailants sprayed them with small arms fire, witnesses said.

Video from the scene showed crowds gathered around the burning vehicles, stoning them and dragging bodies through the streets.

One body was pulled from the burning wreckage and beaten with a stick.

"Fallujah is the graveyard of Americans," they chanted. "We sacrifice our blood and souls for Islam."

Two of the victims' charred bodies were hanged from a bridge over the Euphrates River, with people riding on a donkey cart below cheering. What appeared to be a charred arm also was strung up on the bridge.

The gruesome images -- broadcast around the world -- repulsed and angered many.

U.S. officials are trying to isolate and identify the faces of people seen on the videotapes participating in the abuse of the corpses, and may offer rewards for their capture.

But the initial attackers had their faces covered and may not have participated in the mob.

Paul Bremer, the top U.S. civilian administrator in Iraq, promised that the deaths of the contractors would "not go unpunished."

The four contractors were employees of Blackwater Security Consulting, a contractor providing security for food deliveries, the North Carolina-based company said in a statement.

The company also provides security for Bremer, a senior coalition official said.

Iraqis protest in Baghdad

On Friday, an influential Shiite cleric in Najaf, Iraq, announced his support for two of Israel's biggest enemies -- the Palestinian militant group Hamas and Lebanon-based Hezbollah.

"They can consider me their striking hand in Iraq whenever there is a necessity and whenever there is a need," Moqtada al-Sadr said in a sermon at Al Kufa mosque.

Tens of thousands of demonstrators marched through the streets of Baghdad, chanting anti-American slogans, in protest of the closure of a Baghdad newspaper.

Protesters walked toward the Convention Center, where the coalition holds many of its activities. They set up a stage near the center, where Sheikh Hazim al-A'ragr, imam of Khadimiyah Mosque, led Friday prayers.

The streets leading up to the center were packed with people shouting: "Long live Sadr," "America and the Governing Council are infidels," and "Allow us to fight America."

The cleric has been increasingly outspoken since Hamas founder Ahmed Yassin was killed by Israel last month.

Israel considered the wheelchair-bound Yassin a promoter of terrorism, responsible for many killings.

Al-Sadr said that even though Yassin "has killed a large number of people this does not justify the way he was brutally murdered."

Hamas is on the U.S. State Department's list of terrorist organizations.

Al-Sadr also referred to the U.N. Security Council's failure to issue a resolution condemning the attack, saying, "this proves that all it is, is a tool in the hands of America."

The United States vetoed the resolution because the Council would not include a statement condemning Hamas.

"Iraq and Palestine share one destiny. I support them totally. As the believers are one," he said.

"I wouldn't suggest either one of them should feel secure. I wouldn't propose that any insurance company give them coverage," the prime minister was quoted as saying. "Anyone who kills a Jew or harms an Israeli citizen, or sends someone to kill Jews, is a marked man. Period."

Hezbollah is a militant Islamic group that Israel considers a terror organization.

Al-Sadr condemned the United States and its presence in Iraq, asserting that "America does not want peace in Iraq or any place else."

"Let all be fully aware that those war lovers, the Americans, have brought with them no good over the past year where there have been many problems and events. Only God can end this."

Continued violence

A U.S. soldier and a Marine were killed in separate attacks by suspected insurgents in Iraq, the U.S. military said Friday.

A roadside bomb killed a 1st Armored Division soldier and wounded another when their unit was on patrol in the Al Mansour district of Baghdad Friday morning.

The wounded soldier is in stable condition at a military hospital.

The military said "enemy action" resulted in the death Thursday of a Marine assigned to the 1st Marine Division in the restive Al Anbar province -- which includes Fallujah and Ramadi.

No other details were available.

More than 600 U.S. troops have died since the war began.

Other developments

The chief of the al-Kufa provincial police and his deputy were gunned down Friday in Najaf, the city's police chief said. Saeed Taryak and his deputy were leaving Taryak's house in the Haye Al-Adala district of the city. They were headed to work when insurgents in a blue Oldsmobile ambushed them.

The United Nations High Commissioner for Refugees on Friday issued a news release expressing concern about the security situation in Iraq and said it was "not promoting [refugees to] return to Iraq." The agency also urged countries hosting refugees not to encourage, or force them to return to Iraq at this time.

A trade fair that had been planned for next week in Baghdad was postponed because of security concerns, the Iraqi-American Chamber of Commerce and Industry announced Thursday. The Destination Baghdad Expo -- a scheduled conference of foreign companies interested in investing in Iraq -- was set to begin Monday, according to Raad Omar, a spokesman for the group said.